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Thermochromic behavior of yttrium substituted bismuth oxides Xi Liu, Anne Staubitz, and Thorsten M. Gesing ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11450 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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ACS Applied Materials & Interfaces

Thermochromic behavior of yttrium substituted bismuth oxides Xi Liu†, Anne Staubitz§, ‡, * and Thorsten M. Gesing†, ‡,*

Authors Addresses: † University of Bremen, Institute for Inorganic Chemistry and Crystallography, Leobener Strasse 7, 28359 Bremen, Germany. § University of Bremen, Institute for Organic and Analytical Chemistry, Leobener Strasse 7, 28359 Bremen, Germany. ‡ University of Bremen, MAPEX Centre for Materials and Processes, Bibliothekstrasse 1, 28359 Bremen, Germany. Abstract High-temperature thermochromic materials are poorly explored in fundamental research, let alone applied research, although these materials may be used as low-cost, intuitively usable sensing materials in an industrial environment. Yet, only few of these materials have been described systematically. We describe a series of yttrium substituted bismuth oxides (Bi1-xYx)2O3 (0.05  x  0.25) that show thermochromic behavior with a color change from yellow at low temperatures to various brown hues at high temperatures. The compounds were analyzed between 293 K and 1050 K by X-ray powder diffraction (XRPD), UV/Vis spectroscopy, and differential scanning calorimetry (DSC). A combination of derived absorption spectral fitting (DASF) and Tauc methods were applied to determine the band gap energies and band gap types from the diffuse UV/Vis spectra, respectively. Two types of materials were found: one, with x = 0.05 that exhibits the tetragonal phase at room temperature, and the defect fluorite-type cubic δ-phase at temperatures above 920 K. This phase showed a reversible, gradual color change upon heating, followed by an abrupt color change at the phase-transformation temperature. The second type of material had higher yttrium contents (x > 0.10); these samples were cubic at room temperature and showed a continuous color change upon heating and cooling. In contrast to the material with x = 0.05, these latter phases show a reduced cycle stability and were gradually annealed to the hexagonal phase-I. The samples with x = 0.10 provided a mixture of the - and δ-phase, showing both, the reversible behavior for the - to δ-phase transition and the irreversible behavior concerning the 2-phase. This points the way towards smart materials that can not only sense the actual thermal stress, but also monitor cumulative thermal stresses over a certain material lifetime.

1. Introduction Thermochromic materials change their color in response to a change in temperature, either reversibly or irreversibly. Reversible thermochromes may be used in temperature sensing applications that indicate the current temperature; irreversible thermochromes may have uses for the indication that a given temperature limit has been exceeded. The technological advantage would be that they are intuitive to read out, without little or no additional technology. Perhaps most importantly, they allow sensor/material integration and thus belong to the class of smart materials.1 Although the phenomenon of thermochromism has been reported over a century ago,2-4 most of the applications are restricted to organic compounds and metal complexes, mostly in a dissolved state. In the solid state of organic compounds, there are much fewer thermochromes. The color of those compounds arises in most cases from structural colors such as photonic liquid crystals,5 although some isolated examples of thermochromism based on isomerization reactions in the solid state have been described.6 In solid state inorganic chemistry, there are even fewer examples. Most prominently are the tungsten oxides,5, 7

vanadium oxides and their doped solid solutions and derivatives.8-17 Vanadium oxides and substituted vanadium oxides together with their composites are by far most intensely investigated materials for their thermochromic behaviors including its tunability and repeatability.18 Such materials have even been commercialized in thermochromic windows that turn dark on sun-light irradiation. However, all these materials, organic and inorganic alike, have in common that they are active at relatively low temperatures; well below 600 K in most cases. Temperature-dependent color changes in inorganic solid materials, especially metal oxides are of interest, because such materials may offer the possibility of temperature indication at substantially broader temperature ranges and up to higher temperatures. The mechanisms behind thermochromic behaviors are diverse and of high complexity; only those relevant to metal oxides will be discussed briefly. Examples can be found for temperature dependent change of oxidation states (e.g. ZnO19, 20), change of crystal field (Cr doped Al2O3),21 a change in the crystal structure (e.g. CuMoO4 and its W doped derivatives),22, 23 band-gap shifts of semiconductors with enhanced electron-

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phonon coupling (e.g. 1 nm CuO quantum dots),24 dilatation of ionic bonds (e.g. yttrium iron garnet),25 charge transfer of ions (e.g. rare-earth iron garnets Sm3-xBixFe5O12),26 or metalinsulator transition (e.g. VO2,27, 28 perovskite type substituted manganese oxides).29, 30 In many cases, the color changes originate from a combination of several of these effects, but oftentimes, the cause for the color change remains unexplored.25, 26, 31-33 In terms of bismuth containing thermochromic materials, several complex iodo-bismuthate anions have been prepared that show a pronounced thermochromism between yellow and orange in a temperature range from

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100 K to 294 K or different shades of red.33,34 In that case, the thermochromism was attributed to a change in the band gap of this semiconductor due to a changing Bi…Bi distances. Our research focuses on exploring solid-state materials that exhibit thermochromic properties with the purpose of broadening the application temperature range as well as increasing the diversity of colors. In this study we report the significant and fast (within minutes) thermochromic behaviors observed in yttrium substituted bismuth oxides (Bi1-xYx)2O3 (0.05  x  0.25).

Table 1 Thermodynamic stable phases and polymorphs of (Bi1-xYx)2O3. Phase

Crystal

Space

Metric parameters a

name

system

group

a /pm

b /pm

c /pm

α

monoclinic

P21/c

583

814

748

β

tetragonal

P42/nmc

776

δ

cubic

Fm3m

560

ε

orthorhombic

Pbnb

496

hexagonal

R3m

~400

II

cubic

I213

~1100

III

triclinic

P1 / P1

850

IV

hexagonal

P61(5) / P61(5)22

2287

I / 2

x b range in α /°

β /°

γ /°

67

1002

(Bi1-xYx)2O3 0

36

0 – 0.10

37,38,39

0 – 0.28 MLT, HT*

38,39,40

559

0 M*

42

~2700

0.215 - 0.24

41, 43

0.315 - 0.35 C*

40

0.475 - 0.49

40

0.57 - 0.58

40

566

1274

Ref.

857

111

1904

106

94

*HT: high-temperature phase, LT: low-temperature phase, M: metastable, C: Ln2O3 C-type structure.44 a Only the rough dimensions are given as an overview. For more details see the respective references. b Only ranges of pure phases or polymorphs are given.

Yttrium substituted bismuth oxides are known to crystalize in different structures depending on the substitution level and the thermal conditions like temperature and annealing time. Table 1 outlines some details about the stable and meta-stable phases observed in earlier studies. The pure Bi2O3 have α and  stable phases in the low and high temperature regions.36 On addition and with increasing of Y3+ content, the -type tetragonal phase (up to about 10%)37,38 appears. It is followed by the defect fluorite  phase, which can be maintained down to room temperature by quenching but is only stable at high temperatures if annealed. At low temperatures, for compounds with substitution level between 10 to 28%, the crystals sluggishly transfer into a hexagonal phase (labelled as type I/2 in Table 1), when heated at 923 K for over 100 hours.41 This type I/2 phase is thermodynamically more stable than the defect fluorite  phase at temperatures below 993 K. It appears, but reverts quickly when it is heated above this temperature according to the combined differential thermal analysis (DTA) and X-ray diffraction (XRD) studies.41,43 Watanabe also reported types II, III and IV phases observed in this system at higher content of Y in the solid solution, but for these little information has been documented besides the structure details given in Figure 1 and listed in Table 1.

A series of yttrium substituted bismuth oxides (substitution level of 5% to 25%) offers an excellent opportunity to study the contributions of mechanisms to the color change because there are several advantages in this system: First, there are two solid solution regions separated by the initial substitution level of 10% with two different polymorphs but at the same time similar nominal compositions; Secondly, on heating, compounds with x  0.10, showing the -type phase, will undergo a phase transition into the phase of those with x > 0.10 and then reverse this transition on cooling.44 On the other hand, phases with yttrium contents of x > 0.10 remain in the same polymorph along the whole temperature range between room temperature and their melting points; Thirdly, like the parent compound of the solid solution, α-Bi2O3, these compounds are also semiconductors with absorption edges in the visible light range (ca. 400 to 700 nm); this feature causes their colored appearance to the naked eye. The monoclinic semiconductor α-Bi2O3 itself is of a light yellow color at room temperature, which correlates to its estimated indirect band gap of 2.8 eV.46, 47 Its solid solutions with other metal oxides are often colorful,48, 49 therefore becoming more and more popular nowadays for the applications in novel environmentally friendly inorganic pigments50-52 e.g. to replace toxic compounds like minium


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(Pb3O4), cadmium red (CdSe-CdS solid solutions), lead chromate yellow (PbCrO4), and cadmium yellow (CdS). Low-level substitution of yttrium oxide in bismuth oxide was initially employed to stabilize the cubic -Bi2O3 phase. This had the aim of extending the high ionic conductivity from above 900 K into lower temperature regions (e.g. room temperature) as the pure cubic -Bi2O3 only exists in a narrow temperature range between 1002 K and its melting point occurs at 1097 K.53, 54 Extensive research has been carried out by various groups to investigate the correlation between the local defect structure and high conductivity present in the Y3+ substituted system.55 Maintaining the highly defected cubic -phase from high temperature down to room temperature by this method proved possible within the solid-solution range of 0.1  x  0.25.56 However, the cubic phase in this system is a metastable phase which slowly transfers into the hexagonal phase-I during prolonged annealing at intermediate temperatures (e.g. 100 hours at 923 K).40 The crystal structure of the composition of x = 0.0667 has been determined in the tetragonal space group P42/mnc using neutron powder diffraction.40,45

Figure 1. Phase diagram of (Bi1-xYx)2O3 reproduced after Watanabe.38 The thermochromic behavior of the solid solution reported in this work are exclusively for the samples in the as synthesized state directly after quenching. Therefore, they were in the -phase for samples of x  0.10 and -phase for that x  0.10. 2. Experimental Section 2.1 Synthesis. Five compositions of (Bi1-xYx)2O3 (x = 0.05, 0.10, 0.15, 0.20, and 0.25) were prepared using a conventional solid-state synthesis approach: For each composition, stochiometric ratio of Bi2O3 (Sigma-Aldrich), 10 µm, 99.9%) and Y2O3 (Johnson Matthey Alfa Product, 99.99%) were thoroughly ground in a pair of agate mortar and pestle. For the solidstate reaction, a gold crucible was used, as Bi2O3 reacts with most of other crucible metals and metal oxides. All samples were heated at 923 K and 1073 K for 24 hours each in a

muffle furnace with interval grinding for 20 minutes in between, before being quenched to room temperature in air. A further heating at 1123 K for 6 hours was carried out for the sample with 10% Y3+ substitution on the cation site (x = 0.10) before quenching again in pursuit of a pure phase.38 2.2 Color Observations. The colors of each sample were observed through a quartz glass window in a high-temperature Linkam stage, where the temperature of each sample was adjusted with an estimated standard deviation of 2 K. Photographic images of the samples were taken at temperatures from 300 K to 1050 K using a DigiMicro Profi microscope (dnt, Germany) equipped with 8 white light LED lamps. A white balance was applied to the images. The heating and cooling rate was 30 K per minute and the equilibration time 5 minutes before taking the photos. The color-change observations were made in one heating and cooling cycle. 2.3 X-Ray Diffraction. X-ray powder diffraction (XRPD) data were obtained for all samples on an X’Pert Pro diffractometer (PANalytical, Almelo, the Netherlands) in Bragg-Brentano geometry equipped with Ni-filtered CuKα1,2 radiation Kα1 = 154.05929(5) pm, Kα2 = 154.4414(2) pm) and an X’Celerator multistrip detector. Room-temperature data were collected in the 2θ range 15 - 120°, in steps of 0.0167° with an effective scan time of 24 seconds per step. XRD data at elevated temperatures from 300 K up to 1050 K with 100 K step were collected on samples located in an HTK1200N heating chamber (Anton Paar, Graz, Austria) using the same instruments and settings. Rietveld refinements were carried out for all powder diffraction data using GSAS suite of programs57, 58 and DIFFRAC PLUS TOPAS V4.2 (Bruker AXS GmbH, Karlsruhe, Germany) using the fundamental parameter approach for the refinements. The fundamental parameter set was obtained by fitting instrumental parameters against a LaB6 standard reference material and has been verified by Si standard reference material. 2.4 Thermal Analysis. Simultaneous thermogravimetry (TGA) and differential scanning calorimetry (DSC) measurements were carried out using a TGA/DSC 3+ instrument (Mettler Toledo, Switzerland). For each measurement 30.0(4) mg powder was used and contained in a corundum crucible, while an empty corundum crucible was used as reference. Three cycles of heating and cooling between 298 K and 1073 K were carried out at the heating/cooling rate of 10 K per minute for each sample, after a comparable blank measurement. All thermal analysis experiments used synthetic air as method gas using a flow rate of 20 mL/min. 2.5 Diffuse UV/Vis Spectroscopy. Diffuse UV/Vis reflectance spectra were collected from 290 to 850 nm in step of 1 nm on a UV-2700 spectrophotometer (Shimadzu, Japan) equipped with a UV/Vis DiffuseIR cell (Pike Technologies, USA). The baseline corrections were carried out against BaSO4 powders. For comparison, the spectrum of commercial α-Bi2O3 powder was also recorded.



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For high-temperature measurements, a heating Pike Technologies UV/Vis DiffuseIR heating chamber (Pike Technologies, USA) with quartz-glass window was employed. The spectra were collected from 300 K to 1050 K at an interval of 50 K. The wavelength range was restricted from 290 to 750 nm for measurements at elevated temperatures, because above 1023 K the blackbody radiation gradually shifted into the visible-light region and caused high-level data noise. UV/Vis spectroscopy is one of the most commonly used techniques to analyze the optical properties of semiconductors, especially those of which the band gaps lie in the range below 3 eV. Tauc59 and DASF48 method have been used to extract the band gaps from the spectra of the studied compounds. The Tauc relationship is the classic way to extrapolate the band-gap widths from the slope of the Kubelka-Munk transformation 60 of the reflectance spectra:

naked eye as a fast response between 300 K to 1000 K. The colors of all samples gradually darkened upon heating and lightened upon cooling. At 1050 K, the samples appeared to glow due to the black body radiation. The CIE Lab color codes61 have been extracted from all photos presented in Figure 2 and summarized in Table 2. The samples of x > 0.10 showed a similar level of color change at the respective same temperatures. Although the color of a solid with x > 0.10 appeared to darken gradually, all the colors seemed to still exhibit three regions of hues. When the mean distances were calculated for x = 0.25 using CIE Lab coordinates in one unit space (eq. 3) as an example, the difference between neighboring colors for the temperature measurements within 300 K and 500 K were below 10, from 600 K to 1000 K between 11 to 18, while between 1000 K to 1050 K they were again below 10 (i.e. 7).

𝛼(ℎ𝜈) ≈ 𝐵(ℎ𝜈 ― 𝐸𝑔)𝑛 (1) with the extinction coefficient   F(R), h the Planck constant,  the frequency of incident light, Eg the band gap energy in eV, and n indicating the nature of transition (n = 2 for direct and n = ½ for indirect). Using the DASF equation the absolute value of the band-gap width could be determined:

∆𝑇1 ― 𝑇2 = (𝐿𝑇1 ― 𝐿𝑇2)2 + (𝑎𝑇1 ― 𝑎𝑇2)2 + (𝑏𝑇1 ― 𝑏𝑇2)2

𝑑{ln [𝐴(𝜆)/𝜆]}

𝑛

= (1/𝜆 ― 1/𝜆 ) (2) 𝑔 The combination of both mathematical data treatments is the key to determine the band gap width as well as the type of the transitions (direct or indirect), as detailed and shown by Kirsch et al.: These assumptions hold if complex behaviors (e.g. Urbach tailing) in the electronic band structure are present in the solids.48 𝑑(1/𝜆)

This means that the color changes more significantly in the intermediate region. Interestingly, this three regionbehavior can also be observed for the conductivity of this compound: in the corresponding Arrhenius plot, 53 there are two linear regions with a large transition area in between. The most abrupt color change was observed for (Bi0.95Y0.05)2O3 (x = 0.05) between 900 and 1000 K upon heating. A sudden change was also observed for the sample with nominal composition x = 0.10 in the series, although less pronounced than that for x = 0.05 and in a lower temperature range, between 800 and 900 K.

3. Results and discussion 3.1 Color Changes. The colors of all samples from 300 K up to 1050 K are displayed in Figure 2. At 300 K, the initial colors (assynthesized samples) of samples of x ≥ 0.15 were an almost identical a bright yellow and darkened slightly as x decreased. When x ≤ 0.10, the initial colors of the samples of lowest level of Y3+ substitution (Bi0.95Y0.05)2O3 were orange. When the composition reached x = 0.10 the sample color darkened significantly (from bright yellow to orange) indicating a narrowing of the band gap. This color dependence on the composition indicated the possibility of tuning the color / temperature correlation by varying the amount of substituent. All compounds studied in this paper exhibited significant reversible thermochromic behavior, which was seen by the

Figure 2. Thermochromic behavior of (Bi1-xYx)2O3 solids during heating.

Table 2 CIE Lab codes a of the colors observed in (Bi1-xYx)2O3 from 300 to 1050 K. 300 K L a b 66 12 5 6

400 K L a b 63 18 5 1

0.10

79 0 48

73 5 57

0.15

88 4

82 5

x 0.05

-5 8

-1 7

500 K L a b 69 16 4 1 67 13 5 7

600 K L a b 63 16 2 8 58 14 4 9

82 2 61

72 8 55

(3)

700 K L a b 54 15 2 2 46 12 3 7 56 12 3 7

800 K L a b 53 14 1 2 43 13 3 0 51 15 3 0


900 K L a b 52 17 1 6 32 11 1 3 44 11 2 1

L

1000 K a b

L

1050 K a b

34 3

5

35 0

33 7

6

35 12 5

39 8 10

38 6

3

6

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ACS Applied Materials & Interfaces 89 -7 83 -5 81 -2 50 13 47 13 39 15 37 16 69 5 56 55 6 47 5 2 6 0 6 3 4 4 4 0 2 7 2 4 90 -6 86 -7 82 -3 53 10 41 17 36 17 32 16 0.25 68 8 56 55 8 48 6 5 6 6 6 4 4 0 3 4 2 5 1 9 a L is the lightness coordinate (L = 0 is black, L = 100 is white); a is the red-green oriented coordinate (-a means greenness, +a means redness); b is the blue-yellow oriented coordination (-b means blueness, +b means yellowness). 0.20

3.2 Crystal Structures. XRD patterns (Figure 3) showed that for the samples at room temperature, as synthesized (in black) samples corresponded to the state prior to heating/cooling cycles, and after one heating/cooling cycle (in red). The XRD data indicated that all the samples returned to or remained in the same average crystallographic structures at room temperature. For x > 0.10 the solids were of the face centered cubic (fcc)  phase. The reflections of XRD pattern of the sample (Bi0.95Y0.05)2O3 can be indexed to a single phase of a tetragonal unit cell also known as -type bismuthates.39 Datta et al.38 have reported that a single fcc -phase was achieved for (Bi0.90Y0.10)2O3 after two times heating (first 1073 K for 24 hours, then 1123 K for 6 hours). However, our XRD pattern of sample with 10% substitution level (x = 0.10), which is at the solid- solution limit, still exhibited a mixture of the fcc  phase and the β bismuthates phase despite of the further heating at higher temperature of 1123 K for 12 hours.

The defect fluorite structures of the -(Bi0.75Y0.25)2O3 were previously investigated using a total scattering approach, where reverse-Monte-Carlo (RMC) calculations were carried out using powder neutron diffraction data to model the defects on the O ion sites.55 Slight changes in the preferred ordering of vacancies on the O ion sites were observed at different temperatures, however this did not seem to correspond or be related to the color changes. A multi-phase model of tetragonal -type (Bi0.90Y0.10)2O3 and -(Bi0.90Y0.10)2O3 was employed to fit the XRD data collected for the solid sample x = 0.10 at temperatures lower than 900 K. At room temperature, the phase ratio of β- to -phase in this mixture was about 1:1 from the refinement against the XRD data. Due to the limited sensitivity of X-ray data for light elements (i.e. O2-) in a system that is populated with heavy metal Bi3+ cations, the occupancies and thermal displacement parameters of O2- were left unrefined. The values stated in the literature for the refinements, where the values have been calculated from neutron powder diffraction data on comparable compounds.

Figure 3. Room temperature XRPD patterns of the (Bi1xYx)2O3. After one thermal cycle only, the patterns remain unchanged.

Fitted profiles of Rietveld refinements using the room temperature XRD data of samples of x = 0.05, 0.25, and 0.10 are displayed in Figure 4, while their crystal and refinement parameters, and refined structural parameters are summarized in Tables 3 and 4, respectively. For the refinements, various models were tested for the XRD data of the sample x = 0.05 as all reports in the literature pointed to a tetragonal structure but suggested different modifications. The -type (Bi0.95Y0.05)2O3 was the best fitting model, which closely relates to both -Bi2O362, 63 and -Bi2O3.45 The oxygen position O(3) of this crystal structure was reported to be exclusively associated with doping cations,64 therefore for the refinements, the site occupancy for O(3) was recalculated accordingly. Unlike the pure -Bi2O3, where all sites are fully occupied, there was a high level of defects caused by the O(1) and O(3) position vacancies 39.



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Thermal variations of the cubic - phase lattice parameters for all samples are are plotted in Figure 5. A non-linear thermal expansion of the cubic phase lattice (see Fig. S3 for x = 0.2 as an example) was apparent for all samples. The lattice expanded more pronouncedly on heating with increasing substitution level. The lattice parameters of the cubic phase of this solid solution series obeyed Vegards’ law at every temperature. 3.3 Thermal Analysis. The DSC curves of the 3rd cycles are plotted in Figure 6 for all samples. The onset temperatures of the endotherms and exotherms are labelled in the graph. The thermogravimetric data measured simultaneously did not show any mass loss for any of the samples in the measured ranges. The samples of x ≥ 0.15 presented no thermal events on either heating or cooling at the scanning rate of 10 K per minute. A single endothermic event was evident on heating and one

Figure 4. Rietveld plots (room temperature) for samples of x = 0.05 (upper), 0.25 (middle), and 0.10 (lower) in (Bi1-xYx)2O3.

In the electronic supporting material, the high-temperature XRD data are given in Figures S1 and S2 for samples  0.10, where the reversible phase transitions of β-type   were evident on heating and cooling. The phase transition temperature correlated well with the abrupt color change observed in the sample of x = 0.05. Table 3 Crystal and refinement parameters for (Bi1-xYx)2O3 (x =0,05, 0.10 and 0.25) at room temperature. nominal composition phase ratio crystal system space group unit cell dimensions /pm volume / 106 pm3 Z density / gmcm-3 R factors 2 no. of variables no. of reflections no. of profile points used

x = 0.05 (Bi0.95Y0.05)2O3 100 % III tetragonal P42/n m c a = 776.145 (10) c = 565.676 (10) 340.764 (12) 4 8.848 (1) Rwp = 0.0861 Rp = 0.0648 Rex = 0.0976 RF2 = 0.0977 2.704 39 308 6283

x = 0.10 (Bi0.90Y0.10)2O3 56.92 (78) % III 43.08 (78) %  tetragonal cubic P 42/n m c 𝐹𝑚3𝑚 a = 780.005 (26) a = 553.815 (25) c = 558.154 (26) 339.585 (26) 169.861 (23) 4 2 8.879 (1) 8.641 (1) Rwp = 0.0835 Rp = 0.0615 Rex = 0.1111 RF2 = 0.0657 2.550 35 308 30 6176

x = 0.25 (Bi0.75Y0.25)2O3 100 %  cubic 𝐹𝑚3𝑚 a = 549.5533 (44) 165.970 (4) 2 8.116 (1) Rwp = 0.0822 Rp = 0.0603 Rex = 0.1049 RF2 = 0.0498 2.224 31 30 6283

Table 4 Refined atomic parameters for (Bi1-xYx)2O3 (x =0,05, 0.10 and 0.25) at room temperature. x = 0.05 -type

x = 0.10 -type



x = 0.25 

Atom

Wyc.

Occ

x

y

z

Bi/Y O(1) O(2) O(3)

8g 16h 4d 4c

0.95/0.05 0.475 1 0.10

0.25 0.5338(18) 0.25 0.25

0.00608(24) 0.0932(21) 0.25 0.75

0.48692(36) 0.3254(29) 0.644(7) 0.3110

Bi/Y O(1) O(2) O(3) Bi/Y O(1) O(2) O(3)

8g 16h 4d 4c 4a 8c 32f 48i

0.90/0.10 0.45 1 0.8 0.90/0.10 0.4863 0.0647 0.0008

0.25 0.5428 0.25 0.25 0 0.25 0.3932 0.50

0.0011(9) 0.0590 0.25 0.75 0 0.25 0.3932 0.17

0.4957(11) 0.2797 0.6496 0.3110 0 0.25 0.3932 0.17

1268 (35) 308.7 308.7 308.7

Bi/Y O(1) O(2)

4a 8c 32f

0.75/0.25 0.553(16) 0.048(4)

0 0.25 0.395(6)

0 0.25 0.395(6)

0 0.25 0.395(6)

379(3) 390(56) 390(56)


Uiso/eqiv/pm2

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Anisotropic displacement Parameters /pm2 x = 0.05 Atom U11 U22 Bi/Y 429(23) 229(21) -type O(1) 176 405 O(2) 346 124 O(3) 430 760 x = 0.10 Atom U11 U22 Bi/Y 222(40) 427(45) -type O(1) 176 405 O(2) 346 124 O(3) 430 760

U33 318(6) 157 204 160 U33 516(14) 157 204 160

exothermic event on cooling for both samples (x  0.10) that contained a β-type bismuth yttrate phase. These events were consistent with the reversible tetragonal  cubic phase transitions observed in the temperature dependent XRD data. The transition temperature from the tetragonal phase to cubic phase of Bi0.95Y0.05O3 was at 918 K, consistent with that of Bi0.933Yb0.067O3 (912 K). However the reverse phase transition occurred at ca. 40 K above for Bi0.95Y0.05O3 than the latter one.45

Figure 5. Lattice parameters of the cubic phase of (Bi1-xYx)2O3 at room temperature, 700 K, 800 K, 900 K, and 1050 K. Error bars for the standard deviation were smaller than the symbols. Dashed lines were linearly fitted linear trend lines of the data points at each temperature. For x = 0.05, there is no cubic phase below 900 K.

U12

0 52 0 0 U12 0 52 0 0

U13

0 166 0 0 U13 0 166 0 0

U23 -48(14) 30 0 0 U23 -273(26) 30 0 0

Figure 6. DSC curves of the 3rd cycles for (Bi1-xYx)2O3 (0.05  x  0.25). Onset temperatures of observed events were labelled in the plot. Temperature axis is plotted with sample temperature values.

The transition of the tetragonal phase to cubic phase appeared to take place at a much lower temperature for the bi-phasic sample Bi0.90Y0.10O3. It could potentially be benefiting from the assistance of nucleation of the high temperature phase by the existing secondary phase of the cubic Bi0.90Y0.10O3. From the integrated areas of the weight normalized DSC curves for the two phase-transitions, the amount of β- to the cubic -phase turned out to be about 1:1 in the Bi0.90Y0.10O3, which was consistent with the results from the XRD refinements. The phase segregation of Bi0.90Y0.10O3 into dual phases started at a temperature of about 86 K higher than the pure tetragonal phase Bi0.95Y0.05O3 on cooling45. The DSC measurements were carried out 50 cycles for all samples, and the curves stayed almost identical from the 3rd cycle to the last cycle for each sample respectively. This indicated that no phase transition from the - to type Iphase happened during the 50 times cycling between room temperature and 1173 K. The XRD results on the cycled powder showed identical patterns to those before cycling. This is to confirm that the phase ripening of the type I was unrelated to the observed colors in the samples as heating time is comparable to that in the DTA cycling. 3.4 Optical Properties. Room-temperature reflectance spectra of all samples together with that of α-Bi2O3 are displayed in Figure 7. The spectra of the samples containing various levels of Y3+ substituent and defects clearly showed different characteristics than those of pure bismuth oxide. Pure bismuth oxide exhibits a sharp absorption edge, which has a long linear region (a direct band gap of 2.86(1) eV, an indirect band gap of 2.71(1) eV, and the band gap value from DASF method 2.81(3) eV supporting the experimental results of refs.65 and 66). Contrary to this sharp absorption edge, the reflectance spectra of our samples decreased much more gradually over larger wavelength ranges. This is characteristic for the Urbach-tailing effect that is attributed to the defects in the structure.67 Such an effect was especially prominent in samples of x  0.10, the cubic phase of which contains high level of vacancies on the anion sites. The absorption edges of Y3+ substituted samples appeared at higher wavelengths. The reflectance spectra of the three samples with x > 0.10 were very similar with parts of the



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spectra for x =0.20 and 0.25 almost overlapping. The absorption edge of compound containing the least Y3+ (x = 0.10) was at the highest wavelength. The absorption edges of the sample containing both tetragonal and cubic phases were in between those of spectra for single cubic phase and single tetragonal phase samples. These values were consistent with the color observation of the colors for samples in the as-synthesized state, where the colors of samples with x > 0.10 were very close and in a lighter yellow comparing to the orange colors of samples with x  0.10. In recent years, the modifications of Bi2O3 as with substitution of various cations were proven to lead to interesting semiconductor properties such as luminescence, and visible-light-driven photocatalysis.47, 68, 69 As a semiconductor, the band gap of Bi O 2 3 based compounds at room temperature generally lie below 3 eV, i.e. they present an absorption edge in the visible light range. This leads to the appearance of the typical yellow color for these materials. The diffuse reflectance UV/Vis spectra of our Y substituted solids showed very different feature at the absorption edges than that of the commercial α-Bi2O3. The decrease of α-Bi2O3 reflectance starts at about 450 nm, falls off rapidly and ends at about 430 nm. This results in a clearly separated straight region on the edge. Those of the spectra for Y substituted samples, on the other hand, all exhibited a gently curved edge over a broad wavelength span. On comparison of the crystal structures of all compounds, the high disorder level

Figure 8. Tauc-plots of UV/Vis spectra of (Bi1-xYx)2O3 (0.05  x  0.25) at room temperature with direct (top) and indirect (bottom) modes. The inset in the direct plot gives an example for how the band gap is extrapolated from the spectra in both direct and indirect methods.

Figure 7. Diffuse UV/Vis spectra in reflectance mode of (Bi1xYx)2O3 (0.05  x  0.25) in as-synthesized state and of α-Bi2O3 (monoclinic, commercial) at room temperature.

from anion site vacancies was the common denominator for - and - bismuth yttrate, but it was different to the α-Bi2O3. We speculate that the defects in the substituted solids generate several electronic band structures, which have band gaps close to each other as average structure remain the same. Such disorders created by the Urbach effect in the spectra render the edges not as defined as the crystals without defects. Figure 8 (upper and lower) shows the Tauc plot of the spectra of studied samples in Figure 7 using direct and indirect model, respectively. The band gaps were obtained by extrapolating the longest region in the edge in each mode in the way displayed in the inset in the direct mode for x = 0.05.


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Figure 9. DASF of UV/Vis spectra of (Bi1-xYx)2O3 (0.05  x  0.25) at room temperature.

The room-temperature band-gap values of all samples from direct and indirect modes, and DASF spectra, as given in Figure 9, are summarized in Figure 10. Judging by the values of evaluated band gaps, only direct band gaps were in the reasonable ranges for the colors observed by considering the fact that the band gaps were ca. 2.8 eV and ca. 2.2 eV for α-Bi2O3 (light yellow)65, 66 and α-Fe2O3 (dark red)70, respectively.

Figure 11. Temperature dependent reflectance UV/Vis spectra of (Bi0.95Y0.05)2O3 (top) and (Bi0.75Y0.25)2O3 (bottom).

Figure 10. Band gaps of (Bi1-xYx)2O3 (0.05  x  0.25) at room temperature.

DASF plots of these spectra (Figure 9) are normally used to assist distinguishing the mode of band gaps. It is not easy to tell which mode our samples have, as more than one maximum is observed in the DASF plots. This is not unexpected judging by the curved edges seen in our data. It possible that there is a co-existence of both modes, and the maxima reflect the distribution of the local defect structures.

Figure 12. Direct band gaps (DASF fit) of (Bi0.95Y0.05)2O3 and (Bi0.75Y0.25)2O3 on heating from 300 K to 1050 K.

Temperature dependent UV/Vis spectra in reflectance were plotted in Figure 11 for x = 0.05 and 0.25 upon heating as representatives for the samples that were alike. The data quality of the reflectance data using the heating chamber was lower than that measured using the room temperature setup because of the interference and absorption of the silicate glass window. The side effect from the set-up caused the change of reflectance levels on the higher absorbing regions to be so significant that it prohibited quantitative characterization of the band gaps of the compounds at



elevated temperatures. Nevertheless, qualitatively, gradual red shifts were evident for both samples towards higher temperatures. At 1050 K, the signals became very noisy due to the black body radiation moving from infrared region into the visible region. The evaluated band gaps (DASF) are presented in Figure 12 for these spectra. The band gaps narrowed for both samples on heating, while a sudden drop was observed for sample x = 0.05 at the place where the transition of tetragonal → cubic took place in this compound. 4.

Conclusions

The series of (Bi1-xYx)2O3 (0.05  x  0.25) solid samples showed fast responsive reversible thermochromic behavior, with a large contrast in colors at room temperature and temperatures above 900 K. A gradual color change was observed for all samples in this solid solution series in the whole temperature range. Samples that undergo a phase transition (x = 0.05 and 0.1) exhibited an additional abrupt color shift at the temperature regions of this phase transition. All samples in the system (Bi1xYx)2O3 (0.05  x  0.25) showed a gradual shift of the absorption edge of the UV/Vis spectra towards longer wavelength direction and thus lower energies at higher temperatures. This could be attributed to a continuous narrowing of the band gap in the visible light range, resulting in the gradual color change brought about by heating and cooling. Nevertheless, abrupt color changes were associated with the phase changes in the system. The color differences observed for different samples at the same temperature demonstrate that it is possible to tune the colors at specific temperature via varying the level of substitution. The discoveries reported in this paper pioneer the development of smart pigments that are environmentally friendly, significantly thermochromic, and stable across a broad temperature range. Furthermore, it is possible to create a material, that is both, acutely and reversibly thermochromic as well as cumulatively thermochromic: The reversible color change was attributed to a reversible phase change from tetragonal to cubic, whereas the irreversible change stemmed from a gradual decomposition of the metastable cubic to the hexagonal phase. The separation of these two processes and their use as indicators for the materials’ thermal history and presence will be the subject of further studies. The gradual color change observed for samples in this yttrium substituted bismuth oxides can potentially be of importance to produce a full temperature region indicator between 300 and 900 K.

ASSOCIATED CONTENT Supporting Information. Temperature-dependent X-ray diffraction pattern are provided additionally. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (TMG) * E-mail: [email protected] (AS)

ORCID

Xi Liu: Anne Staubitz: Thorsten M. Gesing:

Page 10 of 29 0000-0002-0785-4557 0000-0002-9040-3297 0000-0002-4119-2219

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

Funding Sources The authors thank the Central Research Development Fund (CRDF) of the University of Bremen for an exploration project grant.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT TMG gratefully acknowledge Central Research Development Fund (CRDF) of the University of Bremen for financial support for the exploration project “Development of High-Temperature Inorganic Thermochromic Compounds”. We are grateful to the German Science Foundation (Deutsche Forschungsgemeinschaft, DFG) regarding the large scientific instrument program for support of the project INST144/435-1 FUGG.

ABBREVIATIONS DSC, differential thermogravimetry.

scanning

calorimetry;

TGA,

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“Table of Contents”


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Figure 1. Phase diagram of (Bi1-xYx)2O3 reproduced after Watanabe. 86x72mm (220 x 220 DPI)



Figure 2. Thermochromic behavior of (Bi1-xYx)2O3 solids during heating. 250x190mm (96 x 96 DPI)


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Figure 3. Room temperature XRPD patterns of the (Bi1-xYx)2O3. After one thermal cycle only, the patterns remain unchanged. 87x54mm (220 x 220 DPI)



Figure 4. Rietveld plots (room temperature) for samples of x = 0.05 (upper), 0.25 (middle), and 0.10 (lower) in (Bi1-xYx)2O3. 84x52mm (220 x 220 DPI)


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Figure 5. Lattice parameters of the cubic phase of (Bi1-xYx)2O3 at room temperature, 700 K, 800 K, 900 K, and 1050 K. Error bars for the standard deviation were smaller than the sym-bols. Dashed lines were linearly fitted linear trend lines of the data points at each temperature. For x = 0.05, there is no cubic phase below 900 K. 88x58mm (220 x 220 DPI)



Figure 6. DSC curves of the 3rd cycles for (Bi1-xYx)2O3 (0.05  x  0.25). Onset temperatures of observed events were la-belled in the plot. Temperature axis is plotted with sample temperature values. 88x57mm (220 x 220 DPI)


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Figure 7. Diffuse UV/Vis spectra in reflectance mode of (Bi1-xYx)2O3 (0.05  x  0.25) in as-synthesized state and of α-Bi2O3 (monoclinic, commercial) at room temperature. 87x57mm (220 x 220 DPI)



Figure 8. Tauc-plots of UV/Vis spectra of (Bi1-xYx)2O3 (0.05  x  0.25) at room temperature with direct (top) and indirect (bottom) modes. The inset in the direct plot gives an example for how the band gap is extrapolated from the spectra in both direct and indirect methods. 87x58mm (220 x 220 DPI)


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Figure 8. Tauc-plots of UV/Vis spectra of (Bi1-xYx)2O3 (0.05  x  0.25) at room temperature with direct (top) and indirect (bottom) modes. The inset in the direct plot gives an example for how the band gap is extrapolated from the spectra in both direct and indirect methods. 87x57mm (220 x 220 DPI)



Figure 9. DASF of UV/Vis spectra of (Bi1-xYx)2O3 (0.05  x  0.25) at room temperature. 87x57mm (220 x 220 DPI)


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Figure 10. Band gaps of (Bi1-xYx)2O3 (0.05  x  0.25) at room temperature. 87x57mm (220 x 220 DPI)



Figure 11. Temperature dependent reflectance UV/Vis spec-tra of (Bi0.95Y0.05)2O3 and (Bi0.75Y0.25)2O3. 88x57mm (220 x 220 DPI)


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Figure 11. Temperature dependent reflectance UV/Vis spec-tra of (Bi0.95Y0.05)2O3 and (Bi0.75Y0.25)2O3. 88x57mm (220 x 220 DPI)



Figure 12. Direct band gaps of (Bi0.95Y0.05)2O3 and (Bi0.75Y0.25)2O3 on heating from 300 K to 1050 K. 87x57mm (220 x 220 DPI)


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TOC Figure


Thermochromic behavior of yttrium substituted bismuth oxides | ACS

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